Various electrical apparatus such as circuit breakers, motor controllers, monitors, motor starters and similar apparatus have many common features. Basically, these apparatus rely upon sensing electrical variables such as line to neutral voltage, line-to-line voltage, phase current, frequency and the like as inputs, either alone or in combination, for ultimately causing a desirable electrical function to occur. For instance circuit breakers sense the current and open upon the occurrence of the predetermined amount of overload current. Motor controllers are utilized to start motors to relay information about the status of one motor from one station to another, etc. Monitors provide readout information about the status of line currents and voltages, power frequency, etc. The state of the art has advanced to a position where the aforementioned provide current voltage, power frequency information, etc. to a printed circuit card which may have surface mounted components, discrete components, transformers, inductors, capacitors, operational amplifiers and all of the appropriate interconnecting paths as well as computer devices such as communication networks, A to D converters, counters, etc. In many applications, microprocessors having random access memories (RAM) and electrically erasable programmable read only memories (EEPROM). Part or all of these devices may be contained in large scale integrated circuits or combinations of large scale integrated circuits or discrete components. Many of the aforementioned contained potentiometer devices such as three-point or two-point variable resistors, variable capacitors, adjustable operational amplifiers, etc.
It has long been recognized that in order for any of the devices or apparatus described previously to operate reliably and accurately, it is necessary for the sensing transformers, internal circuit board components, large scale integrated circuits, etc. to accurately depict both within the basic circuitry and at the output devices such as readout devices, circuit breaker tripping devices, relaying devices, those variables which are monitored or read. For instances, the circuit breaker is programmed to trip within a specific period of time at 10 amperes, for example, and if in fact 10 amperes is the amount of current flowing in the line to be protected but the gain adjustment and offset factors and conditions within the circuitry indicate that only 9.9 amperes are being read, then an error exists which in some instances could lead to catastrophic consequences. In the past, provision has been made for eliminating the error by placing the device in question in a test mode whereby a precision value of a desired variable is provided as a sensed input to the system and various potentiometers, offset adjustment devices and the like are manipulated or "tweaked" at the end of the production process so that desirable occurrences happen at the exact value of input variable at which they are supposed to happen. Generally, after this has been completed, a protective coating of material is placed over the adjustment features so that they may not be tampered with by subsequent purchasers, users, etc. One can see that this leads to a number of disadvantages or problems. One disadvantage lies in the fact that the process immediately described as highly labor intensive and furthermore requires a great deal of judgement, experience and perhaps even dexterity on the part of the calibrator or adjustor. Furthermore, in some instances where a microprocessor is employed, a resistive capacitive network rather than a crystal oscillator may be utilized for determining a microprocessor's time base. In sensitive applications, this time base accuracy may not be sufficient. In addition, problems can arise if the "board" to be calibrated is small or has been conformal coated.
Two areas in which calibration problems are likely to arise are associated with analog sampling systems that require both gain and offset. Gain adjustments are required for precision circuits requiring operational amplifiers to magnify or attenuate a signal before being processed by an analog-to-digital converter. Offset adjustments are made to remove errors caused by DC biasing currents existing in a circuit. Those offsets could be present on the input signals or generated by operational amplifiers in the circuit. Gain adjustment is usually made by imposing a multiplying factor on a signal whereas offset adjustment is made by adding or subtracting a constant to or from the signal being processed.
U.S. Pat. No. 4,550,360 issued on Oct. 29, 1985 to J. J. Dougherty entitled "Circuit Breaker Static Trip Unit Having Automatic Circuit Trimming" teaches one way to overcome problems associated with the prior art.
The Dougherty patent describes the use of a microprocessor and selected input values to correct for gain and offset. However, emphasis is directed to correction for individual components or a class thereof within the system. It requires testing individual components or classes of individual components using different inputs such as, for example, full scale current for current transformers and five milliamperes for a diode and then deriving a microprocessor memory correction value related thereto.
However, it would be advantageous to be able to calibrate the entire system using only a single input where the calibration is related to the ultimate output value rather than to the individual values of components. It would be further advantageous to be able to achieve the foregoing without adding extra component elements to the system being calibrated and to do the calibration in a non obtrusive manner.
It can be seen, therefore, that the present mode for calibration has many disadvantages as described previously. It would be desirous therefore if an advantageous calibration procedure on apparatus could be found which was relatively inexpensive, required a minimum of human intervention, was highly accurate, highly reliable and as close to being automated as possible.